[OpenSER-Users] Beep in audio stream.

Hi, I'm currently setting op an openser server. My setup at the moment is as follows: registrar (pstn) <=> yate (sip server) <=> openser <=> sip_phone. As I make a call with the sip phone to a pstn line, the rtp stream is forwarded from the yate server to openser, which acts as en media proxy with rtpproxy. During the call I get very annoying beeps every 2 or 3 seconds. The beeps sound a bit like cost-beeps or somethin. When I connect the phone directly to the yate server however, which then starts acting as media-proxy, I do not get any beeps. Furthermore, if I remove force_rtp_forward() from openser config, it stops being proxy for the stream, but still I get these annoying beeps. Excluding any problems with rtpproxy. Clearly, the registrar sends these beeps, but he doesn't send them when I connect with yate. Am I missing something that can trigger this behaviour? Thanks, Marc Dirix